Glass sheet heating such as is necessary for tempering was primarily conducted for many years in the past by the "vertical" process wherein tongs are utilized to suspend the glass sheet for conveyance through a furnace in which the heating is performed. One problem with the vertical process is that the tongs tend to leave marks in the glass. Another problem with the vertical process is that the suspended glass sheet upon softening tends to distort due to its own weight and thereby loses optical quality.
Another type of glass sheet heating system developed in the 1960's is the gas hearth type wherein glass sheets are conveyed over a thin film of gas supplied through a lower hearth having a planar surface with a slight tilt. At the lower side of the tilted hearth, a conveyor engaged with the edge of the glass is provided in order to move the glass during the heating. While gas hearth conveyors have been used for many years, the cost involved to operate such systems is significant and thus not totally satisfactory.
While there is prior art disclosing the use of roller conveyors for glass sheet heating during tempering, there was no significant commercial acceptance of roller conveyors for glass sheet tempering furnances before the 1970's. This was due to problems involved in maintaining planarity of the conveyed glass sheets and uniformity in roll driving so as not to mar the conveyed glass surfaces.
In the early 1970's, glass sheet tempering systems having roller conveyors with friction drives were introduced into the glass tempering industry and received immediate recognition that has since resulted in such systems being used throughout the world. That type of friction drive roller conveyor for use in glass sheet heating is disclosed by the U.S. Pat. No. 3,806,312 of McMaster and Nitschke. The system involved includes a horizontal roller conveyor having rolls on which the glass sheets are conveyed during the heating. The rolls of the conveyor span the gap between a pair of spaced horizontally extending surfaces over which driving reaches of a pair of continuous drive loops are respectively moved to support and frictionally drive the ends of the rolls. Both support surfaces are located within the heating chamber of the furnace and the drive loops utilized are solid steel belts which can withstand the high temperature present upon passage through the heating chamber. During operation of this drive mechanism, the driving reaches of the continuous drive loops are maintained taut so as to maintain the upper surfaces of the rolls in a single plane along which the glass sheets are conveyed over the rolls. Also, a coupling between a pair of drive sheaves that respectively drive the pair of continuous drive loops at one end of the system and adjustable supports for each of the sheaves provide coordinated driving thereof in a manner that ensures conveyance of the glass sheets in the direction intended with minimal lateral drift.
A second generation of glass tempering systems having frictionally driven roller conveyors was subsequently developed as disclosed by the U.S. Pat. Nos. 3,934,970 and 3,947,242 of McMaster and Nitschke. This second generation included a furnace whose roller conveyor has the basic type of frictional drive mechanism discussed above but with ends of the conveyor rolls projecting outwardly through side slots defined between side walls of upper and lower housing portions of the furnace. The continuous drive loops of the drive mechanism are driven over external support surfaces to support and frictionally drive the roll ends and thus operate at a much lower temperature than is the case when located within the heating chamber. The lower temperature involved with this external location facilitates the use of continuous chains for the drive loops as opposed to the solid steel belts that are utilized when the drive loops pass through the heating chamber. These chains have teeth that permit positive driving thereof by toothed sheaves associated therewith as opposed to depending on frictional wrap force about the sheaves as with the solid steel belts. Also, the chains can bend to a much smaller radius of curvature than the solid steel belts and thereby permit the use of smaller diameter sheaves.
Each of the glass sheet tempering systems described above with the frictionally driven roller conveyors was initially manufactured as the continuous type wherein the glass is conveyed in a single direction throughout its heating. Such continuous systems have substantial output that may be greater than needed for any particular factory installation, especially if the width of the conveyor is sufficiently wide to handle the large size glass that has found widespread use in the recent past for architectural purposes.
Another glass tempering system as disclosed by the U.S. Pat. No. 3,994,711 of McMaster was introduced after the systems described above and had the same friction drive mechanism but with the conveyor of the furnace driven in an oscillatory manner so as to oscillate a glass sheet or sheets being heated independently of a roller conveyor of the associated quench unit where the glass is subsequently tempered. During an index cycle, the furnace and quench unit conveyors are coupled to provide coordinated conveyance of a heated glass sheet or sheets from the furnace into the quench unit. A first electric motor drive mechanism of this system drives a roller conveyor of a load station as well as the roller conveyor of the furnace while a second electric motor drive mechanism drives a roller conveyor of an unload station as well as the roller conveyor of the quench unit. During oscillatory driving of glass being heated within the furnace and independent oscillation of glass being cooled within the quench unit, the roller conveyors of the load and unload stations are respectively uncoupled from the first and second electric motor drive mechanisms so as to permit loading of glass to be tempered at the load station and unloading of tempered glass at the unload station. The oscillatory driving of the furnace roller conveyor considerably shortens the length of the total system while still conveying the heated glass sufficiently fast to prevent sagging thereof between the spaced rolls of the conveyor.
Other glass sheet heating conveyors of the frictionally driven roller type are disclosed by U.S. Pat. Nos.: 4,133,667; 4,233,053; 4,341,546; 4,356,912; and 4,512,460.
Frictional driving of glass sheet roller conveyors has also been accomplished as disclosed by U.S. Pat. No. 4,617,043 with an endless belt that is pressed by spring loaded rolls against the ends of the furnace rolls. As disclosed by U.S. Pat. Nos. 4,297,121 and 4,300,937, glass sheet heating conveyors of the frictionally driven roller type have also utilized an endless loop that is movable over support rollers and on which the conveyor rolls are supported with resilient sleeves on the conveyor roll ends for reducing vibration.
As disclosed by U.S. Pat. No. 4,332,608, glass sheet heating systems have also been constructed with cradled roll conveyors wherein support rolls cooperate in pairs to support and frictionally drive opposite ends of conveyor rolls on which the glass sheets are conveyed during the heating. Such frictional driving has also previously utilized O-rings that increase the friction generated to provide the roll driving.